1. Field of the Invention
The present invention relates to a method for processing a load, and, more particularly, to a method for steam processing porous loads.
2. Description of the Prior Art
In order to effectively sterilize porous loads, such as fabric packs, the air trapped within the interstices of the load must be withdrawn prior to the stabilization phase of the sterilization cycle so that the steam can contact every portion of the load. Air pockets within the load block the entrance of steam. Furthermore, air leaks within the sterilization chamber are detrimental to sterilization.
Means for detecting the presence of air in the chamber during the sterilization phase and method of removing air from the chamber and the load prior to the sterilization phase have been suggested. For example, Henfrey U.S. Pat. No. 3,402,991, (the "Henfrey Patent") which issued on Sept. 24, 1968, describes a load simulator which simulates the conditions in the sterilization chamber, specifically, the steam-air proportions within the chamber. The Henfrey load simulator includes a temperature sensing means which responds to the steam-air conditions and actuates controls which reduce the period of sterilization. The load simulator described by the Henfrey Patent operates on the premise that even in those instances where there is an air leak, that sterilization of the load will eventually occur if a certain temperature is detected by the temperature sensing means in the load simulator. Until a predetermined temperature is sensed the sterilization phase continues.
Young et al. U.S. Pat. No. 4,164,538, (the "Young Patent") which issued on Aug. 14, 1979, and is incorporated herein by reference, describes a method for conditioning the sterilization chamber prior to the sterilization phase of the cycle. A sequence of cyclic pressure pulses is described wherein the chamber is alternately evacuated to both a minimum time of at least one minute and a minimum pressure of at least ten inches Hg, and then pressurized to a level below the desired sterilization pressure. The Young Patent states that minimal fabric loads may require longer or deeper vacuum treatment for air removal during conditioning because the pressure drop in the chamber is rapid. Full fabric loads on the other hand, according to the Young Patent, require a longer exposure time to bring the vacuum down to the ten inch Hg level.
It has been observed, however, that full fabric loads can retain more air, even when the chamber is evacuated to ten inches Hg over a relatively long period of time, than is retained by minimal fabric loads when the chamber is evacuated over a shorter period to a level of ten inches Hg or less. For purposes of detecting unacceptable levels of air in the chamber during the sterilization phase, therefore, the proportion of steam to air will not be the same for full and minimal loads at the start of the phase.
Following the cyclic pressure pulses of the conditioning phase, the sterilization method described in the Young Patent proceeds by introducing and maintaining steam at a desired level for an effective time. During the sterilization phase, the ambient chamber temperature is constantly monitored by a temperature probe in the drain line. If air remain in the fabric load, the temperature of the load will be lower than the ambient temperature in the chamber for a given point in time. Although the load temperature rises, it lags behind the ambient chamber temperature. Thus, there is a lag between the temperature sensed by the probe and the actual temperature of the load. Because the length and accuracy of the sterilization cycle are often dependent upon the temperature sensed by the chamber drain line probe, the cycle may be concluded before the load has been subjected to the sterilizing temperature for a full cycle.
There is a need for a more thorough method of removing the air from porous loads to substantially eliminate the disparities in the steam-air proportions in a chamber between minimal and full porous loads. There is a further need to substantially eliminate the lag between the temperature sensed and the actual load temperature. Finally, there is a need for detecting even small quantities of air in a chamber which can compromise the effectiveness of the sterilization phase.